Due to a growing market demand for automotive vehicles that are fuel efficient and environmentally friendly, automotive vehicle manufacturers increasingly are devoting a substantial portion of their research resources on fundamentally new technologies. Much of this research has focused on the internal combustion engine, which is used in the vast majority of automotive vehicles currently produced.
Although the internal combustion engine is inexpensive, reliable, easy to refuel and provides the desired performance, it is desirable to reduce the consumption of fossil fuels and emissions of these engines. To further address these concerns, manufacturers are also focusing their research on other areas of the automotive vehicles, such as braking systems and other drivetrain components.
As a result of this research emphasis, the automotive industry has developed a number of alternative drive systems for powering automotive vehicles. Generally speaking, a range of different concepts have been developed for automotive drive systems. At one end of this spectrum of available drive systems is the conventional drive system which uses an internal combustion engine that directly drives a standard automatic or manual transmission. At the other end of the spectrum is the electrical vehicle concept. Electrical vehicles operate completely on electrical energy stored on board, but generated elsewhere from fossil fuel or other sources. Typically, the drive system of an electrical vehicle uses a large electrical drive motor for torque generation and large capacity batteries for electricity storage. Some critics of conventional internal combustion engine drive systems prefer the concept of electrical vehicles because the electrical drive motor emits no polluting exhausts. Electrical automotive vehicles have generally been unsuccessful in the marketplace, however, because they can only travel short distances before the batteries must be recharged. In addition, the recharging process usually lasts several hours.
In response to the disadvantages of electrical vehicles, manufacturers of automotive vehicles have developed the concept of hybrid drive systems. This drive system typically includes both an internal combustion engine powered by fossil fuel and an electrical motor powered by electricity. The goal of hybrid drive systems is to combine the advantages of conventional internal combustion engine drive systems with the advantages of electrical drive systems. Thus, the optimal hybrid drive system desirably is capable of traveling long distances with good drive performance while requiring only a short amount of time to refill with fuel or recharge the batteries. Additionally, the drive system may be fuel efficient and environmentally friendly.
The concepts of hybrid drive systems are generally defined by two categories. In one category, referred to as high storage hybrids, the electrical drive system acts as the dominant system and the internal combustion engine provides supplemental power when needed. These systems typically include a large electrical motor and large capacity batteries similar to an electrical vehicle but also include a small internal combustion engine. The internal combustion engine provides additional power when extra acceleration is desired and can be used to generate electricity for longer distance travel. In the other hybrid category, referred to as low storage hybrids, this combination of drive systems is reversed. The internal combustion engine acts as the dominant system and the electrical drive system provides supplemental power. In this type of system fuel efficiency is increased by using a smaller internal combustion engine than is typically used in conventional automotive vehicles. However, drive performance remains similar to conventional drive systems since the electrical drive system provides assist power when needed. The electrical drive system can also be used in a regeneration mode to divert torque from the drivetrain to generate electricity for recharging the batteries. Low storage hybrid drive systems may be more readily acceptable to consumers as an alternative to conventional drive systems. One reason for this acceptance is that consumers typically demand drive performance and vehicle behavior equal to or similar to what they have experienced with current automotive vehicles.
Typically, an integrated starter-generator (xe2x80x9cISGxe2x80x9d) is used for the electrical drive system in low storage hybrid drive systems. Several different versions of ISGs are available; but generally speaking, the ISG is connected to the drivetrain of the automotive vehicle between the internal combustion engine and the wheels. Accordingly, the ISG is usually capable of functioning like a motor to generate drive torque from electricity stored in the batteries. Alternatively, the ISG is able to generate electricity from drive system torque. Thus, at least four different modes of operation of the ISG are possible. In the first mode, the ISG supplies torque to the engine to turn the crankshaft during starting of the engine. The ISG acts like a conventional starter in this mode; therefore the need for a standalone starter is eliminated. In the second mode, the ISG diverts some of the torque produced by the engine during normal operation in order to generate electricity. The electricity is then used to recharge the batteries and for powering the various electrical components used throughout the automotive vehicle. In this mode the ISG acts similarly to a conventional alternator, thus eliminating the need for a standalone alternator. In the third mode, the ISG draws electricity from the batteries to supply torque to the drivetrain during heavy loading. This mode enhances drive performance of the automotive vehicle by improving acceleration or allowing the engine to operate at lower average speed and higher average load for improved thermal efficiency. In the fourth mode, the ISG generates electricity from torque supplied by the drivetrain. This mode is sometimes referred to as regenerative braking or regenerative deceleration. In effect, this mode allows the automotive vehicle to recapture energy that is normally lost by conventional drive systems during deceleration, or slowing, of the vehicle.
Several problems are commonly encountered with the regenerative deceleration mode in currently available low storage drive systems. For example, the ISG causes the automotive vehicle to decelerate at an inconsistent rate between different deceleration events of the vehicle. This problem occurs because the torque applied by the ISG changes depending on the amount of electricity stored in the batteries and the electricity being used by the vehicle""s electrical components. When the batteries are very low and capable of receiving a lot of electricity, the ISG applies more torque to generate more electricity. When the batteries are fully charged, the ISG applies very little torque, if any, for electricity generation. This variance in torque is undesirable because the driver can not predict the rate at which the vehicle will slow down.
Another problem is that the ISG typically produces an unfamiliar deceleration behavior. In conventional drive systems, deceleration of the vehicle is provided by either wheel brakes or from compression braking when the drivetrain rotates faster than the equilibrium speed of the engine. Compression braking commonly occurs when the driver lets off of the gas pedal or when the vehicle is coasting down a hill. In these situations, compression is produced in the engine cylinders of the internal combustion engine by the rotating pistons, thereby resulting in a consistent and predictable slowing of the automotive vehicle.
Hybrid drive systems employing regenerative braking are currently unable to produce a deceleration behavior that is similar to conventional drive systems. The deceleration torque experienced by the driver no longer varies predictably depending on vehicle speed, engine speed and transmission setting as inherently happens in conventional drive systems. Like the inconsistent deceleration behavior that results from varying electricity demands, consumers can find this difference between hybrid and conventional drive systems inconvenient and disconcerting. The deceleration behavior of hybrid drive systems is also complicated further by the fact that the internal combustion engine, which continues to be used as the dominant power source, also produces deceleration due to conventional compression braking in addition to the deceleration produced by the ISG.
Accordingly, a control system is provided for decelerating a vehicle at a predictable rate while optimizing regeneration of the deceleration torque. The control system receives input data, including a desired deceleration torque, an integrated starter-generator (xe2x80x9cISGxe2x80x9d) torque capacity, and a compression torque capability of an internal combustion engine. The control then changes a setting of the ISG and a setting of a variable valve timing system (xe2x80x9cVVTxe2x80x9d) to achieve the desired deceleration torque. The torque applied by the ISG is maximized and the compression torque of the engine is minimized to increase efficiency of the regenerative deceleration mode.